The principles of MRI are set forth in several patents such as U.S. Pat. No. 5,304,933, which is incorporated herein by reference. MRT, sometimes referred to as interventional MRI or intraoperative MRI, is the performance of an interventional medical procedure on a patient in an MRI system. During the procedure, a surgical instrument is inserted into a patient in order to perform the procedure at a predetermined site in the body. The MRT system is used in this case to monitor in quasi real-time the correct placement of the instrument and also to observe the nature and the extent of the effect of the intervention on the tissue.
In an MRI and/or MRT system a strong uniform magnetic field is required in order to align an object's nuclear spins along the z-axis of a Cartesian coordinate system having mutually orthogonal x-y-z axes. The required strong uniform magnetic field, used for full body imaging, is normally in the order of 0.1 to 2 Tesla. The image quality and the accuracy of an MRI and/or MRT system is dependent on the degree of uniformity of the strong uniform magnetic field. Uniformity is critical in MRI and/or MRT applications because if the strong uniform magnetic field is not properly uniform within the volume of interest, the desired discrimination between different elements, due to the finely controlled magnetic field gradient, will be subject to misinterpretation. Typically, the uniformity required for the strong uniform magnetic field is within the order of 10 ppm within the volume of interest. It is essential for MRT systems used in interventional procedures to be based on an open structure, so as to provide the physician easy access to the intervention site.
Presently, most MRI systems employ a large magnet, which effectively surrounds the whole body of the patient, to produce the strong uniform magnetic field. Such magnets are usually large superconductor resistive or permanent magnets, each of which is expensive and heavy. Further, the access to the patient in these cases is obstructed.
Attempts have been made to provide open magnets for interventional procedures by employing two spaced-apart Helmholtz superconductive coil assemblies. They provide only limited space between the assemblies allowing for constricted access by only one person, such as a surgeon. Moreover, they are large, massive, immobile and expensive. See U.S. Pat. No. 5,410,287 (Laskaris et al.) and U.S. Pat. No. 5,428,292 (Dorri et al.).
U.S. Pat. No. 4,875,485 (Matsutani) discloses an apparently more compact configuration, based on a pair of spaced-apart superconductive Helmholtz coil assemblies, arranged for movement relative to a platform carrying the patient. The access to the patient remains restricted in this case as well, due to the additional space occupied by the cryostat. Also, the movement of the coils independently of one another is impractical, because the superconducting properties of the coils require extreme precision in positioning of the two poles, in the absence of which the magnetic system quenches.
In comparison to superconductive systems, permanent magnets are less expensive, generate only a minimal unwanted fringe field and are not involved with liquefied gas handling or vacuum requirements. Open access MRI systems based on permanent magnets have been disclosed by U.S. Pat. No. 4,829,252 (Kaufman) and U.S. Pat. No. 5,134,374 (Breneman). Both are using a pair of opposing magnetic flat circular poles, employed one above the other, with the patient lying down between the magnets. The poles are mounted on end plates, supported by connecting members, which provide return paths for the magnetic flux. These systems are massive and immobile and the access to the patient is encumbered by the supporting structure.
A pair of opposing permanent magnet assemblies for use in MRI, each made of concentric magnetic rings, composed of a set of magnetic polygonal blocks, is disclosed in U.S. Pat. No. 5,332,971 (Aubert). Aubert teaches that the opposing concentric rings within each of the pairs of permanent magnets are to be spaced apart from each other the same distance. The magnet is massive, weighing about 3 tons and is therefore not amenable to movement relative to a patient's body.
In each of the above prior art magnets, used for providing the large uniform magnetic field for MRI and/or MRT applications, the magnetic field is generated in a first stage as uniformly as possible. More uniformity is achieved subsequently by shimming.